I am not necessarily against a Mandarin immersion program but if one is adopted children who already speak Chinese (any dialect) should be excluded.
If fluent Chinese speakers are included it will be impossible for teachers to create a coherent program. Either the program will be geared to the non-Chinese speaking students and the Chinese speakers will be bored or the program will be geared to the Chinese speakers and the non-Chinese speakers will be lost.
If the non-Chinese speaking students drop out they will be replaced by Chinese speaking students and we will end up with a private Chinese school funded by the public school system as opponents of MI fear.
The reason why speakers of Chinese dialects other than Mandarin should be excluded is because the written language is common to all dialects and Cantonese speaking friends assure me that the differences between spoken Cantonese and Mandarin are not all that great.